It would depend on whether you were talking about viability in cases where you take extraordinary measures. I don't know how meaningful it would be to expect someone seeking an abortion to take extraordinary measures to preserve a premature child's life. As it is now, however, with a lot of late term abortions the aborted child accidentally survives, maybe with an arm or a leg cut off, and in many hospitals the custom has then been to set the baby aside until it conveniently dies by thirst, starvation, or exposure, and can be disposed of.
Rape, life of the mother, and incest also could be large loopholes, depending on what the politician's real attitudes are. That was how George Bush described himself during his first campaign, but in office he has been pretty forcefully pro-life, from the very first day.
Life of the mother is very different from health of the mother, if that is really the guideline being followed. I agree with those who argue that abortion is wrong in the case of rape and incest--because in what way is the baby at fault for what was done by others? Why should he be killed? If the mother understandably couldn't deal with the trauma, then the baby could be offered for adoption.